Heretofore, the wet cleaning process called the RCA cleaning process has been mainly conducted for cleaning substrates used as members of electronic products. The RCA cleaning process is a cleaning process using a mixed liquid of sulfuric acid and an aqueous solution of hydrogen peroxide (SPM) heated at 120 to 150° C., a mixed liquid of ammonia and an aqueous solution of hydrogen peroxide (APM) heated at 60 to 80° C. or a mixed liquid of hydrochloric acid and an aqueous solution of hydrogen peroxide (HPM) heated at 60 to 80° C. To overcome the problems arising on application of this process such as the great cost of the chemicals and the cleaning agents in great concentrations, the cost of a great amount of pure water and ultrapure water used for removing the chemicals and the cleaning agents by rinsing, the cost of the air conditioning for removing vapor of the chemicals and freshly supplying the clean air and the load on the environment such as the use of water in a great amount, disposal of chemicals in great amounts and discharge of waste gases, various attempts to simplify the process have been made, and many achievements have been obtained. A typical example of the achievements is the ultrasonic cleaning technology using cleaning water containing a dissolved specific gas such as hydrogen.
It is the essence of the new cleaning technology that a specific gas is dissolved in a great concentration. For maintaining the great cleaning effect with stability, it is necessary the concentration of the dissolved gas be controlled at a value which is great but does not exceed the concentration of saturation under the condition of the use. It is preferable from the standpoint of safety that the concentration of the dissolved gas is kept at a value not exceeding the concentration of saturation since the undesirable possibility that the dissolved hydrogen forms bubbles and remains in the system under the atmospheric pressure increases when hydrogen is dissolved in an amount exceeding the concentration of saturation. In response to the above technological need, a technology in which, after a gas (mainly nitrogen) dissolved in pure water or ultrapure water used as the material water is removed by a degassing treatment, the gas desired to be dissolved is supplied in an amount smaller than the amount to provide the concentration of saturation and dissolved in the material water, has been developed by the present inventors. The usefulness of the above technology has been recognized, and the technology has been widely used as the novel technology for the wet cleaning.
For example, as the process for the wet treatment of articles for the treatment such as substrates for semiconductors and glass substrates for liquid crystal display devices to which extremely clean surface is required and electronic members to which the completely hydrocarbon-free surface at the room temperature is required, a process for the wet treatment of articles for the treatment in which the article for the treatment is treated with ultrapure water containing hydrogen or ultrapure water containing hydrogen and a very small amount of a rare gas under irradiation with ultrasonic wave of 20 kHz or greater, wherein the ultrapure water used as the material water is water degassed in a manner such that the content of dissolved gasses other than the rare gas is at most 10 ppm or smaller, has been proposed (Patent Reference 1).
As the apparatus for supplying ultrapure water containing hydrogen which can supply the ultrapure water for cleaning containing hydrogen to the use point with the stable concentration of the dissolved hydrogen under fluctuation of the amount of the use and does not dispose of the ultrapure water in an excess amount as waste water, an apparatus for supplying ultrapure water containing hydrogen which comprises a water tank of the closed type for holding a mixture of ultrapure water containing hydrogen left remaining without being used in the use point and ultrapure water supplied as the supplement, a pump for transferring water held in the water tank, a degassing portion for removing dissolved gases in the transferred water, a dissolving portion for dissolving hydrogen supplied from a portion for supplying hydrogen into the degassed water and a recycling pipe for recycling water to the water tank via a filter and the use point, and supplies ultrapure water containing hydrogen in an amount necessary at the use point while the ultrapure water containing hydrogen is circulated in the step of cleaning electronic materials using ultrapure water containing hydrogen, is proposed (Patent Reference 2).
In accordance with the above process or by using the above apparatus, the gas-containing cleaning water in which a specific gas is dissolved in an amount which is great but does not exceed the concentration of saturation can be efficiently produced. However, in accordance with the above process or by using the above apparatus, the step of degassing for removing the major amount of the dissolved gas from pure water or ultrapure water used as the material water is indispensable, and a suitable mechanism for decreasing the pressure such as a vacuum pump is necessary.
In the cleaning step in which a great amount of the gas-containing cleaning water is necessary, it is required for saving water that the gas-containing cleaning water after being used (waste water of cleaning) be recycled and reused. A recycling system can be constructed by removing contaminants in the waste water of cleaning and adjusting the concentration of the dissolved gas. In accordance with a conventional process, the degassing treatment is conducted after the waste water of cleaning is cleaned, and the specific gas is dissolved again. In particular, in the case of the system in which cleaning water containing dissolved hydrogen which exhibits a great cleaning effect is recycled, a considerable amount of hydrogen remains dissolved in the water although hydrogen is replaced with the air during the cleaning step and the amount of hydrogen is decreased. For removing the remaining hydrogen, the considerable amount of hydrogen must be discharged through a mechanism for decreasing the pressure such as a vacuum pump, and this procedure causes a problem on the safety. It has been desired that the above problem is overcome so that the cleaning using the gas-containing cleaning water in which a specific gas is dissolved is more effectively utilized.    [Patent Reference 1] Japanese Patent No. 3521393    [Patent Reference 2] Japanese Patent Application Laid-Open No. Heisei 11 (1999)-77021